Environmental constraints on plant transpiration and the hydrological implications in a northern high latitude upland headwater catchment

Vegetation affects water, carbon and energy transfer in the soil-plant-atmosphere system and mediates land-atmosphere interactions by altering surface albedo, roughness and soil macro-porosity, intercepting rainfall and transpiring water from soil layers. Vegetation water use (Ec) is regulated by stomata behaviour which is constrained by environmental variables including radiation, temperature, vapour pressure deficit, and soil water content. The relative influences of these variables on Ec are usually site specific reflecting climate and species differences. At a catchment scale, Ec can account for a large proportion of total evapotranspiration, and hence regulates water storage and fluxes in the soils, groundwater reservoirs and streams. In this study, we estimated transpiration from short vegetation (Calluna vulgaris) using the Maximum Entropy Production model (MEP), and measured sap flow of two forest plantations, together with meteorological variables, soil moisture and streamflow in an upland headwater catchment in northern Scotland. Our objectives were to investigate the environmental constraints on Ec in this wet humid and cool summer climate, and the hydrological responses and regulations of Ec in terms of rainfall and streamflow. Results will assist the assessment of hydrological implications of land management in terms of afforestation/deforestation.